Conventionally, a load current supplied from an AC power source and flowing into a load such as an AC/AC converter includes a component of a harmonic current. In order to solve this problem caused by the harmonic current, an active filter in parallel with the load is provided, so that harmonic components of the load current do not flow out to the AC power source (see, for example, Patent Document 1).
A conventionally known technique reduces the capacity of an active filter to the minimum necessary level (see Patent Document 2). In order to do so, Patent Document 2 shows detecting a generation amount of harmonic components of different orders from a harmonic current generated in a load connected through a system bus to a system power source. It further discloses, with an order harmonic component that is most likely to occur among components of respective harmonic orders as a reference, setting a predetermined mutual ratio for each order harmonic component based on the harmonic component of the referential order. It further discloses performing calculation processing using the generation amount of each order harmonic component and the mutual ratio for each order harmonic component, such that the compensation amount output from the active filter is controlled to be equal to a target amount obtained through subtraction of an upper limit of a harmonic component (standard value) regulated by a harmonic guideline from the generation amount of each order harmonic component.
On the other hand, if an electrolytic capacitorless inverter is employed as a load, the compensation current control of the active filter becomes oscillatory at the resonance frequency of the electrolytic capacitorless inverter, as the power source impedance increases. Therefore, the active filter has to be controlled in order to reduce the resonance. For example, a control device for an active filter subtracts a differential value with an amplification at a certain gain with respect to an installation point voltage of the active filter from a value obtained based on a compensation current and a load current. As a result, the device obtains a voltage command value which is a command value of a voltage to be output from the active filter (see Patent Document 3).